Overvoltage surge arrester with improved voltage grading circuit

ABSTRACT

An overvoltage surge arrester for relatively high voltage applications is provided with a voltage grading circuit comprising a relatively low exponent resistor in parallel with a relatively high exponent resistor, for preventing spurious operation of the arrester under conditions in which the outside surface of the porcelain housing becomes contaminated with a conductive film. Also disclosed is a grading circuit in which a linear resistor is additionally connected in series with the high exponent non-linear resistor of the above circuit.

United States Patent [1 1 Sakshaug OVERVOLTAGE SURGE ARRESTER WITH IMPROVED VOLTAGE GRADING CIRCUIT [75] Inventor: Eugene C. Sakshaug, Lanesboro,

Mass.

[73] Assignee: General Electric Company [22] Filed: Jan. 16, 1974 [21] App]. No.: 433,655

[52] [1.8. CI 317/68, 315/36, 317/70 [51] lnt. Cl. H02h 9/06 [58] Field of Search 315/36; 317/68, 69, 70

[56] References Cited UNITED STATES PATENTS 3,091,721 5/1963 Yost 317/70 3,313,978 4/1967 Miller 315/36 Jan. 7, 1975 3,611,044 10/1971 Osterhout et al. 317/70 Primary Examiner-James D. Trammell Attorney, Agent, or Firm-Volker R. Ulbrich [57] ABSTRACT An overvoltage surge arrester for relatively high voltage applications is provided with a voltage grading circuit comprising a relatively low exponent resistor in parallel with a relatively high exponent resistor, for preventing spurious operation of the arrester under conditions in which the outside surface of the porcelain housing becomes contaminated with a conductive film. Also disclosed is a grading circuit in which a linear resistor is additionally connected in series with the high exponent non-linear resistor of the above circuit.

8 Claims, 7 Drawing Figures PAIENTEUJAH 7:925

SHEET 2 OF 3 mmwwwwo CURRENT llV flMPERt'S OVERVOLTAGE SURGE ARRESTER WITH IMPROVED VOLTAGE GRADING CIRCUIT BACKGROUND OF THE INVENTION The present invention relates generally to electrical overvoltage surge arresters and relates particularly to, but is not limited to, such arresters for use at relatively high voltages and which are provided with a plurality of spark gaps electrically connected in series.

An overvoltage surge arrester for relatively high alternating current voltages of 3 kv (kilovolts) or higher typically includes a number of individual and similar arrester modules stacked in electrical series inside an elongated, hollow porcelain housing cylinder which is closed at both ends by top and bottom metal end caps, respectively. The top cap is in contact with the top of the uppermost module and isconnected to the power line. The bottom cap is in contact with the bottom of the lowermost module and is connected to ground.

Each arrester module includes one or more gaps connected in series with one or more non-linear resistance current limiting elements, or valve blocks. A grading resistor is commonly connected in parallel with the gap of each module to maintain a uniform voltage distribution among the modules. Additional components may be connected in or among the modules to further enhance the operation of the arrester.

The individual modules are designed to normally have across them an operating voltage. Each module also has a characteristic sparkover voltage, somewhat above the operating voltage, at which the gap sparks over to initiate an active operation of the arrester. Typically, the sparkover voltage is on the order of 1.7 times the operating voltage. The operating voltage and the sparkover voltage of an entire arrester are simply the sum of the respective such voltages of the modules which are connected in series in the arrester. Thus, the arrester is designed to have an operating volt age equal to the normal line-to-ground voltage of the line which is to be connected to the arrester.

A serious problem with high voltage arresters of the type described above can arise when the outside sur face of the porcelain housing becomes contaminated with an electrically conductive film of, for example, salt spray or wetted cement dust. Various effects of such contamination are described for example in the following US Pat. Nos.:

2,179,297 issued Nov. 7, 1939 to F. B. Johnson 3,467,936 issued Sept. 16, 1969 to E. Nasser 3,510,726 issued May 5, 1970 to J. E. Harder 3,683,234 issued Aug. 8, 1972 to A. Rodewald 2,688,715 issued Sept. 7, 1954 S. A. Vorts et al. Contamination of the porcelain may cause the arrester to fail by sparking over at the operating voltage, rather than at the appropriate higher sparkover voltage. Such erratic operation frequently results in destruction of the arrester.

For an understanding of the above-described failure mode it is useful to consider the alternating current leakage due to capacitive coupling of the arrester. to facilitate the discussion of such capacitive leakage, there is shown in FIG. 1 of the drawings a schematic representation of a fragment of a prior art arrester 10. The arrester includes three of a number of arrester modules 12 inside a housing cylinder 14 provided with an upper end cap 16. Each module 12 is outlined by a dashed rectangle l3 and includes an electrode gap section 18, two valve blocks 20 to either side of the gap section 18, and a grading resistor 22 connected across the remote sides of the valve blocks 20.

The capacitive coupling associated with each of the modules 12 is attributable primarily to four capacitive components, which are represented in FIG. 1 by an equivalent circuit of dashed lines and dashed capacitors. The dashed lines and capacitors are used here to avoid confusion of the coupling capacitances with actual capacitors which are sometimes included in an arrester.

The first coupling components C C C of the first, second, and third modules, respectively, beginning with the uppermost, are the capacitance of the module 12 itself, including, for instance, the capacitance' between the electrodes of the gap section 18.

The second coupling components C C C of each respective module 12 are the capacitance due to the coupling of the internal parts of the arrester 10 with the housing 14.

The third coupling components C C C of each respective module 12 are the capacitance of the porcelain housing cylinder 14 itself.

The fourth coupling components C C C of each respective module 12 are the capacitance due to coupling of the porcelain housing cylinder 14 to ground.

During normal operation of the arrester 10, the voltage across C and therefore across the gap section 18 of the first module 12, is greater than the voltage across C because C and the grading resistor 22 of the first module 12 must also carry all the capacitive leakage and the normal grading currents for all the other modules 12 down the line. The grading resistors 22 are typically chosen so that during normal operation, the grading current through them is much larger than the capacitance leakage current. Thus, the voltage across each gap is held by the resistors 22 at very nearly the same value as across every other gap. At normal line voltages, the total capacitive leakage current is typically on the order of, for instance, 0.01 milliamperes, while the grading current through the grading resistors 22 is typically on the order of l milliampere.

When the surface of the arrester housing 14 is contaminated with a conducting contaminant, such as salt, cement dust, fly ash etc, and the contaminant is wetted so that it becomes capable of conducting current and initiating discharges on portions of the surface, the currents through capacitances C C C increase considerably. The reason for the increase is that the voltage distribution on the porcelain surface becomes extremely non-uniform at times from rapidly changing non-uniform surface conductivity of the contaminant film due to uneven wetting and the effects of arcing on the wetting patterns. Since the current in a capacitor is proportional to the rate of change of voltage across it, such rapidly changing voltage distributions result sporadically in substantially increased capacitive leakage currents being forced through the grading resistors 22, particularly those nearest the line voltage. Peak leakage currents of tens of milliamperes or higher may be coupled into the arrester 10 under severely contaminated conditions. Such relatively large sporadic capacitive leakage currents through the grading resistors 22 can result in a voltage drop across the grading resistors 22 high enough to cause a spurious sparkover voltage to appearacross the parallel connected gap electrodes the voltage grading under contamination conditions.

However, such. capacitors have not been found to be entirely reliable, largelydue to the difficulty in making the ceramic dielectric sufficiently stable. Because of the risk of capacitor failure, such capacitors are generally connected in series with added resistors to limit breakdown current. Also, the capacitors should have graded values of capacitance, a further complexity. Such an arrangement is relatively costly, while nevertheless remaining somewhat unsatisfactory as grading means where the arrester is subjected to relatively long overvoltage surges, such as switching surges.

Present arresters commonly utilize grading resistors of a material having a non-linear current-voltage characteristic, such as a silicon carbide compound, to lessen the effects of contamination of the housing. The current-voltage characteristic of a resistor is given by the relationship I KV", where I is the current, K is a con- 7 stant and n is the exponent of the voltage. The numerical value of the exponent for such grading resistors is typically 4.5. It is evident that a moderate difference in current through such grading resistors produces a relatively small change of voltage. Therefore, even though the currents through the grading resistors 22 are somewhat different because of the capacitance currents previously described, the voltage across the gap section 18 is reasonably uniform. However, when the surface of the porcelain housing becomes contaminated, the capacitance leakage currents can become so large that sufficient voltage is imposed on some of the gaps to cause unwanted sparkover, even when such non-linear grading resistors are employed.

It might appeaar from the above that a further lessening of the effects of contamination conditions can be achieved by simply reducing the resistance of the grading resistors 22, so that even under severe contamination conditions the current flowing through the grading resistors 22 will remain large compared to the capacitive leakage currents. The changev in current through the resistors caused by changes in capacitive current wouldthen be comparatively small, and the change in voltage across them inconsequential. Increased current through the resistors 22 at operating voltage would, however, result in excessive resistive heating in the arrester. 0n the other hand, if the increased current is provided with low voltage drop across the grading resistors 22 only above a certain value of voltage, spurious sparkover can be avoided without undue resistive heating at the operating voltage. Such a result can be achieved by the use of a higher exponent material for the grading resistors 22.

With the use of certain metal oxide compositions, such as ones containing primarily zinc oxide together with selected impurities, a grading resistor can be made which has a relatively high exponent on the order of about 45, or 10 times the typical exponent of the presently used silicon carbide grading resistors. Composition for high exponent metal oxide resistors are disclosed for example, in the following U.S. Pat. Nos.:

3,689,863 issued Sept. 5, 1972 to Matsuoka et al.

3,598,763 issued Aug. I0, 1971 to Matsuoka et al.

3,496,512 issued Feb. 17, 1970 to Matsuoka et al.

It has been found, however, that such high exponent grading resistor compositions are unstable when subjected to continuous 60 Hz. (hertz) alternating currents at a current density of only a small fraction of an ampere' per square centimeter. Although some compositions of zinc oxide non-linear resistors are more stable than others when subjected to alternating currents, even these more stable compositions become unstable when carrying continuous currents of the magnitudes required for a voltage grading resistor of even an uncontaminated high voltage arrester. Their resistance decreases substantially over a period of time of conducting such currents. Continued flow of current with such decreased resistance will then result in destruction of the resistor. For this reason it has until now not been commercially feasible to use such high exponent material for grading resistors in high voltage arresters.

SUMMARY OF THE INVENTION The novel arrester comprises a gap having in parallel with it both a relatively low exponent grading resistor and a relatively high exponent grading resistor, connected in parallel relationship to one another. With the novel arrester, the problem of the instability of the relatively high exponent material is avoided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side, sectional view of a fragment of a prior art arrester and including a schematic representation of an'electrical circuit therein.

FIG. 2 is a side sectional view of a fragment of an overvoltage surge arrester in accordance with the preferred embodiment of the invention.

FIG. 3 is a side view of an arrester module of the arrester of FIG. 2 seen at with respect to the view of FIG. 2.

FIG. 4 isa side, sectional, partly schematic view of a fragment of the arrester of FIG. 2.

FIG. 5 is a graph illustrating the current-voltage characteristics of grading resistors of the arrester of FIG. 2.

FIG. 6 is a partly sectioned perspective view of a portion of an arrester module in accordance with another embodiment of the invention. I

FIG. 7 is a graph illustrating the current-voltage characteristics of the grading resistors of the arrester module of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT paired side by side, only one pair of which are shown entirely in the FIG. 2. Another view of the modules 28 at 90 orientation with respect to the FIG. 2 is shown in FIG. 3. All the modules 28 of the arrester 24 are similar and have a 6 kv rating, meaning that they are designed to be subjected to an individual operating voltage of about 4.8 kv. Like reference numerals are used to identify like members of the modules 28 of FIGS. 2 and 3.

Referring now to FIGS. 2 and 3, each module 28 includes a gap section, or unit 30 contacted on each of its faces by a valve block 32. Connected electrically in parallel with the series of the gap section 30 and the valve blocks 32 are a high exponent grading resistor 34 and a low exponent grading resistor 36. The modules 28 are series-stacked in pairs which are clamped on insulating spacers 38 between metal support plates 40 facing in opposite directions for connection in series as a pair by a diagonal metal strap 42 extending between two thin metal contact plates 44, each located between the spacer 38 and the valve block 32. The gap electrodes of the gap unit 30 are located inside ceramic supporting discs of the gap unit 30 and are not shown in detail, since a detailed description of their particular configuration is not needed for an understanding of the present invention. The primary current-carrying gaps of the gap unit 30 are connected in series with the two faces of the gap unit 30.

The low exponent grading resistor 36 is around rod of asilicon carbide varistor composition provided with metal end caps for electrical contact. It is about 5, cm (centimeters) long, about 1 .cm in diameter, and has a current-voltage characteristic exponent of about 4.5.

The high exponent grading resistor 34 is a round rod of zinc oxide varistor composition provided with silver metallizing on a short portion of the ends for electrical contact to receiving clips 46 which are mounted on a support plate 40 and the contact plate 44, respectively. It is about cm long, about 1.6 cm in diameter and has a current-voltage characteristic exponent of about 45.

To aid in describing the operation of the arrester 10 of the preferred embodiment, there is shown in FIG. 4 of the drawings a partially schematic representation of the electrical elements of the arrester 10 and the capacitive coupling alternating current leakage components associated with contamination of the arrester housing 26. The capacitive coupling components are represented by the same symbols used in the discussion above relating to FIG. 1, since the coupling components of FIG. 4 are analagous to those of the earlier discussion. The first arrester module 28 at the top is outlined by a dotted rectangle 47. The module 28 includes a valve block 32 electrically connected to each side of the gap section 30. Connected in parallel with the remote sides of the valve blocks 32 is the low exponent grading resistor 36. Also connected in parallel with the remote sides of the valve blocks 32 is the high exponent grading resistor 34.

When the arrester 10 is operating in a steady state at operation voltage, the low exponent grading resistors 36 allow a current of about 1 milliampere to flow through them, such that any capacitance effect due to contamination on external surface of the housing 26 will not be significant at line voltage frequencies. Consequently, the voltage will be relatively uniformly graded over the gap section 30. The current through the high exponent grading resistors 34 is very small, on the order of only microamperes. This is not enough current to present a significant risk of instability failure of the high exponent resistors 22. When contamination of the arrester 10 results in substantially increased capacitive leakage current to be sporadically forced through the grading resistors 34, 36, the bulk of this current is passed through the high exponent resistor 34. Because of the high exponent of the resistor 34, this current can be passed without sparking over the gap section 30, since the resistive voltage drop can be maintained at well below the sparkover voltage.

A graphic representation of the current-voltage characteristics of the low exponent and high exponent grading resistors 36, 34 of the arrester 10 of the preferred embodiment is presented roughly in FIG. 5 of the drawings. The abscissa of the graph corresponds to the logarithm of the current in amperes through the resistor on a logarithmic scale, while the ordinate corresponds to the rated voltage of the arrester module 28. The operating voltage mentioned earlier is generally about percent of the rated voltage. The dashed curve 48 shows the behavior of the low exponent resistor 36. As the current rises, the voltage across the resistor 36 also rises at a relatively rapid rate. The dotted curve 50 shows the behavior of the high exponent resistor 34. The voltage rises relatively slowly with increasing current. It can be seen from the two curves 48, 50 that with the resistors 34, 36 connected in parallel so that they see the same voltage, almost all the current is carried by the low exponent resistor 36 at the operating voltage of 80 percent rating. As the current increases so that the voltage rises to percent the rated voltage, the current becomes evenly divided between the resistors 34, 36. The major portion of higher currents than those at 100 percent rated voltage are carried by the high exponent resistor 34. Thus, the resultant curve for the parallel resistors 34, 36 can be considered for practical purposes to be the combination of that part of the low exponent curve 48 below the high exponent curve 50, together with that part of the high exponent curve 50 to the right of the low exponent curve 48. Thus, the high exponent resistor 34 becomes a significant current carrier only at or above the rated voltage. Since the sparkover voltage of the gap section 28 is generally at least percent of the rated voltage, sparkover of the gap section 28 by spurious leakage currents in the grading resistors 34, 36 is prevented by the high exponent resistor 34.

For applications where the gap sparkover level is considerably higher than the voltage rating of the arrester, such as, for instance, 1.35 times the rated voltage the current passed through the high exponent grading resistors 34 at the sparkover voltage level just prior to the arresting of an overvoltage impulse may become undesirably high. This may result in undue instantaneous heating effects. In such a case, it is desirable to connect in series with the high exponent grading resistor 34 a linear resistor to control the current in this range. An arrester with such a linear resistor is the subject matter of a copending application filed in the name of J. S. Kresge, entitled Overvoltage Surge Arrester With Improved Voltage Grading Circuit, assigned Ser. No. 433,656, filed concurrently with the present application, and assigned to the same assignee as is the present application.

The resistance value of the linear resistor should be chosen so that at the sparkover voltage level, the current in the high exponent resistor and the linear resistor is at about the desired maximum for the zinc oxide grading resistor. For the arrester 10 of the preferred embodiment such a linear resistor may have a resistance value of about 1,000 ohms and a power rating of about 2 watts. FIG. 6 illustrates a portion of an arrester module such as those of FIGS. 2 and 3 in which a clip 52 for securing one end of a high exponent resistor 54 is fastened to an elongated insulating member 56, which in turn is fastened to a contact plate 58 of the module 60. Connected across the insulating member 7 56 and in electrical contact with the clip.52 and the 'contact plate 58 is a linear resistor 62. V

The effect of the linear resistor 62 on the currentvoltage characteristics of the connected grading resistors is roughly illustrated by the graph shown in FIG. 7 of the drawings. The" FIG. 7 is in all respects essentially similar to FIG. 6 except for the addition of the solid line curve 64 representing the current voltage characteristics of the linear resistor 62. It can be seen that just below the sparkover voltage, which in this instance is taken to be about 135 percent the rated voltage, the voltage across the linear resistor rapidly becomes significant to limit the current in the high exponent resistor 54 connected in series with it.

GENERAL CONSIDERATIONS The contamination of arrester housings has become an especially important problem in recent years. As increasingly complex and costly equipment is used in electrical power systems, greater effort is made to protect this equipment against overvoltage surge damage to the insulation. Whereas it was only recently considered adequateto protect .againstovervoltages of 1.5 times the operating voltage or higher, it is now common to require protection against overvoltages of only 1.35 times the operating voltage. Thus, in effect, the protective voltage level tolerance has been tightened on overvoltage surge arresters. Arresters providing protection at levels closer to their operating voltage are much more likely to be affected by contamination of their housings.

While the present invention is most useful for alternating current applications, since that is where zinc oxide material presents a stability problem, an arrester in accordance with the invention is, of course, also useful for direct current applications.

Although the arrester of the preferredembodiment was for high voltage applications and had a plurality of gaps, the present invention may be used for an arrester having any number of gaps, including a single gap, and is useful whether or not a valve section is utilized in the arrester.

' A very significant benefit of the present invention is a substantially decreasedcost in the porcelain housing member for the arrester. Where contamination on the outside of the porcelain poses severe problems, such as forextra high voltage applications, it has been the practice to maximize the surface creeping length of the porcelain with a complex skirt configuration and an increased diameter. With the present invention, however, such features are to a large degree unnecessary. The significance of this benefit lies in that the porcelain member is a very substantial portion of the total cost of an arrester. With the present invention, the housing may be made of a smaller diameter and provided with a minimum of skirts.

While the exponent of the relatively high exponent grading resistor material in the preferred embodiment was as great as ten times the exponent of the relatively low exponent material, it should be understood that the high exponent material need only have an exponent substantially greater than the exponent of the low exponent material to provide the benefits of the present invention. A substantially greater exponent is taken to mean that the exponent is greater by more than the extent to which the exponent normally varies for a given material in the production process. Moreover, the low exponent material is not limited to silicon carbide, but may be any of various non-linear resistance materials which are presently used, or could presumably beused in an arrester for that purpose. For that matter, the relatively low exponent material could in fact be linear, thus having an exponent of unity. For practical reasons, however, silicon carbide and zinc oxide for low and high exponent materials, respectively, are the most likely materials to be used for the foreseeable future.

The instability problem with .zinc oxide materials is known to those in the art of making overvoltage'surge arresters. It has been found, for instance, that a zinc oxide resistor carrying an alternating current of 5 X 10 amperes per square centimeter at 60 Hz may decrease in resistance by as much as a factor of two over a period'of only 2,000 hours. Continued flow of such current will then result in destruction of resistors of this kind. For this reason, a zinc oxide resistor alone as a grading resistor would not result in a reliable product, although in other respects it would alleviate the contamination problem by providing a high grading current at elevated voltages. With the silicon carbide grading resistor connected in parallel with the zinc oxide resistor, the silicon carbide resistor typically carries a current of on the orderof tenths of milliamperes at the normal operating voltage level for the arrester. The current through the zinc oxide arrester at this voltage is only on the order of tenths of microamperes. At such alow continuous current, zinc oxide resistors can remain stable. On the other hand, the current through the zinc oxide arrester at a level of only approximately 1.7 times the normal line voltage is on the order of whole amperes. Since the currents capacitively coupled into the arrester even under severe contamination conditions generally do not exceed several hundred milliamperes at this voltage level, the gap voltages are held at sufficiently uniformly graded values by the present parallel circuit combination. In this regard, it should be noted that of the various compositions which have been described, as for example in the patents referred to above under Background of the Invention" for zinc oxide resistors, some are more stable at a given'temperature than others when subjected to 60 hertz alternating currents. In practicing the present invention, it is desirable to select from the various available compositions and processes for making such resistors those which will yield a resistor sufficiently-stable for the particular conditions to which it will be subjected.

The grading resistors have been described above as being in parallel with the series of the valve blocks and the gap section but actually the presence of the valve blocks between the gap and the grading resistor connection can be ignoredin considering grading currents. Their resistance in this regard is negligible, and becomes significant only at much higher voltages. The grading resistors can be connected in parallel with only the gap or gaps of the gap section without departing from the spirit of the invention. For that matter the nature of that portion of the arrester module across which the grading resistors grade the voltage is largely immaterial, since the grading resistor circuit would in fact alone perform its voltage grading frunction without being otherwise connected across a gap. As a practical matter, however, the grading function is particularly useful for and applicable to a gap, such as in an overvoltage surge arrester, which is subjected to voltages sufficient to result in sparkover.

The description herein of a grading resistor as connected in parallel with a gap section includes arrangements in which valve blocks or various other elements, including other gaps, are additionally connected in series with the gap section and in parallel with the resis- The spark gap section, across which the grading resistors are connected, can include a single spark gap assembly of two gap electrodes or a plurality of gap assemblies, and the gap assemblies may be all of the same type, such as a simple gap, or of a more complex type, such as a current limiting type. They may also be a mixture of different types of gap assemblies connected together in series, parallel, or combinations thereof. The important consideration is that the gap section be essentially a voltage-sensitive switch which closes very suddenly at a predetermined voltage higher than the voltage to which it is subjected during normal operation at the operating voltage of the arrester.

I claim:

1. An electrical overvoltage surge arrester, comprismg:

a housing comprising at least two conductive terminal members spaced apart by a hollow insulating member;

a spark gap section disposed inside said housing, said spark gap section comprising at least one spark gap assembly with a gap electrically connected in series with said terminal members;

a first grading resistor of a first non-linear resistance material electrically connected in parallel with the gap of said spark gap assembly, the degree of nonlinearity of said first material being indicated by a first numerical exponent for the voltage in an equa- '10 tion describing the general current-voltage characteristics of said first material, and

a second grading resistor ofa second non-linear resistance material electrically connected in parallel with said gap and with said first grading resistor, the degree of non-linearity of said second material being indicated by a second numerical exponent for the voltage in an equation describing the general current-voltage characteristicsof said second material,

said first exponent being substantially greater than said second exponent.

2. The arrester defined in claim 1 and wherein said first exponent is at least twice the magnitude of said second exponent.

3. The arrester defined in claim 2 wherein said first exponent is about ten times as great as said second exponent.

4. The arrester defined in claim 3 wherein said first material is substantially a metal oxide.

5. The arrester defined in claim 4 wherein said first material is substantially zinc oxide.

6. The arrester defined in claim 5 wherein said second material is substantially silicon carbide.

7. The arrester defined in claim 6 comprising at least one non-linear resistance valve block element electrically connected in series between said spark gap and one of said terminal members.

8. The arrester defined in claim 7 wherein said spark gap section comprises a plurality of said spark gap assemblies electrically connected in series between said 

1. An electrical overvoltage surge arrester, comprising: a housing comprising at least two conductive terminal members spaced apart by a hollow insulating member; a spark gap section disposed inside said housing, said spark gap section comprising at least one spark gap assembly with a gap electrically connected in series with said terminal members; a first grading resistor of a first non-linear resistance material electrically connected in parallel with the gap of said spark gap assembly, the degree of non-linearity of said first material being indicated by a first numerical exponent for the voltage in an equation describing the general currentvoltage characteristics of said first material, and a second grading resistor of a second non-linear resistance material electrically connected in parallel with said gap and with said first grading resistor, the degree of non-linearity of said second material being indicated by a second numerical exponent for the voltage in an equation describing the general current-voltage characteristics of said second material, said first exponent being substantially greater than said second exponent.
 2. The arrester defined in claim 1 and wherein said first exponent is at least twice the magnitude of said second exponent.
 3. The arrester defined in claim 2 wherein said first exponent is about ten times as great as said second exponent.
 4. The arrester defined in claim 3 wherein said first material is substantially a metal oxide.
 5. The arrester defined in claim 4 wherein said first material is substantially zinc oxide.
 6. The arrester defined in claim 5 wherein said second material is substantially silicon carbide.
 7. The arrester defined in claim 6 comprising at least one non-linear resistance valve bLock element electrically connected in series between said spark gap and one of said terminal members.
 8. The arrester defined in claim 7 wherein said spark gap section comprises a plurality of said spark gap assemblies electrically connected in series between said terminal members. 